Skin-mounted electronics for wearable computing and health monitoring require stretchable circuits that match the mechanical properties of soft natural tissue. Current approaches include so-called “deterministic architectures” in which mechanical compliance is introduced through geometry, for example, PANI or Ag—Ni alloy coated on a woven fabric or thin metal interconnects with serpentine or pre-buckled wavy geometries. Because the conductive materials are intrinsically rigid (elastic modulus ≥1 GPa) and inextensible, stretchable functionality must be engineered through microscale geometric design and cleanroom fabrication.
Another popular approach is to use conductive polymers and composites that are intrinsically soft and deformable. Polyurethanes (PU), polydimethylsiloxane (PDMS), polyacrylates, fluoropolymers, and styrene ethylene butylene styrene copolymer (SEBS) are typically used as the carrier medium. To be conductive, they are typically embedded with percolating networks of rigid metallic nano/microparticles or carbon allotropes (e.g. MWCNT, graphene) or are grafted with polyaniline, ionomers (e.g. PEDOT:PSS) and other conductive polymer groups.
While promising for low-load or moderate-strain applications, these composites are typically more rigid and less elastic than homogenous elastomers, stretchable elastane fabrics, or natural biological tissue. Nonetheless, they have adequate mechanical properties for electronic skin applications and can be patterned using a variety of rapid fabrication methods.
Liquid metal (LM)-based circuits represent a versatile alternative for stretchable electronics that bypass some of the limitations of deterministic architectures and polymer composites. Ga-based LM alloys like Ga—In eutectic (eGaIn; 75% Ga and 25% In, by wt.) and Ga—In—Sn (Galinstan; 68% Ga, 22% In, 10% Sn) are particularly attractive because of their low viscosity, high electrical conductivity, low melting point, low toxicity, and negligible vapor pressure. When encapsulated in a soft elastomer, for example, PDMS, liquid-phase traces of Ga-based alloy can provide highly robust electrical connectivity between solid state elements within a circuit and enable extreme elastic deformability. Another feature of Ga-based LM alloys is that, in O2-rich environments like air, they form a self-passivating surface layer of Ga2O3 (thickness ˜1-3 nm) that dramatically reduces surface tension and allows patterned traces to hold their shape. This oxidation and moldability has enabled eGaIn to be patterned with a variety of techniques based on stencil lithography, selective wetting, reductive patterning, microcontact printing, jetting, and 3D direct-write printing.
Since the mid-2000s, eGaIn microfluidic systems have been engineered for a broad range of applications. In the last couple of years, this includes continued efforts in sensing and electromechanical transducers, force characterization for medical endoscopy, reconfigurable metamaterials and radio antennae that exhibit tunable operating frequency and enhanced range.
Despite their extraordinary potential, progress in LM electronics is currently limited by methods for integration with MOSFETs, microprocessors, chipsets, cable adapters, and other solid-state technologies (SSTs). Recent efforts with so-called dual-trans printing and z-axis conductive elastomer have successfully addressed integration but only with millimeter-scale pins and traces.
Successful integration of LM-based circuits and micro-scale SSTs requires processing techniques that are compatible with conventional PCB manufacturing, enable reliable interfacing between the terminals of the LM circuit and I/O pins of packaged electronics, and allow for planar circuit features with dimensions below 100 μm.
When encapsulated in elastomer, micropatterned traces of Ga-based liquid metal (LM) can function as elastically deformable circuit wiring that provides mechanically robust electrical connectivity between solid state elements (e.g. transistors, processors, sensor nodes). However, LM-microelectronics integration is currently limited by challenges in rapid fabrication of LM circuits and the creation of vias between circuit terminals and the I/O pins of packaged electronics.